Which of the following gases has the highest critical temperature?

The liquified gas that is used in dry cleaning along with a suitable detergent is

An ideal gas cannot be liquefied because ......

The number of statements,which are correct with respect to the compression of carbon dioxide from point $(a)$ in the Andrews isotherm from the following is $..........$.
$A.$ Carbon dioxide remains as a gas up to point $(b)$
$B.$ Liquid carbon dioxide appears at point $(c)$
$C.$ Liquid and gaseous carbon dioxide coexist between points $(b)$ and $(c)$
$D.$ As the volume decreases from $(b)$ to $(c)$,the amount of liquid decreases

An ideal gas cannot be liquefied because ........

Which of the following gases is most difficult to liquefy?